Electromagnetic interference (EMI) filters are commonly used throughout the electronics industry. EMI filters are typically placed on input power lines of electronic equipment so that high frequency electromagnetic noise does not enter or escape the equipment.
It is desirable for EMI filters to attenuate high frequency noise as much as possible. Typically, noise that must be attenuated is in the frequency range of about 10 kilohertz or 150 kilohertz to 30 megahertz. Noise in the frequency range of about 1–30 Mhz is particularly problematic because it can propagate across a circuit board via inductive coupling.
FIG. 1 shows a typical conventional EMI filter circuit. The filter has common-mode capacitors 10 that attenuate common-mode noise. First capacitor 12 and second capacitor 14 are connected in parallel on either side of coupled inductors 16 18. The coupled inductors 16 18 are coupled by a toroidal ferrite core 20. The power input can provide DC power, or 120 volt 60 hz conventional line power, for example. The power output can be connected to a computer, telecommunications device or any other electronic equipment.
In a conventional application, coupled inductance offers common mode attenuation and the leakage inductance offers differential mode attenuation.
A problem with the circuit of FIG. 1 is that the attenuation of the filter decreases with increasing frequency. This degradation in filter performance occurs because of parasitic inductive coupling (i.e., mutual inductance) between filter components. The parasitic inductive couplings allow high frequency signals (e.g., above about 1 Mhz) to jump between components and appear at the filter output. FIG. 2, for example, show plots of insertion voltage attenuation in dB versus frequency for an ideal circuit without parasitic inductive couplings, and for a real circuit. The parasitic inductive couplings cause an enormous degradation in filter performance that increases with increasing frequency.
With modern electronics operating at ever-increasing speeds and ever-decreasing voltages, it is becoming more difficult to protect sensitive circuits from damaging and disruptive electrical noise. Consequently, EMI filters must be improved to provide increased attenuation of electrical noise, particularly at high frequencies.
It would be an advance in the art to provide an EMI filter having increased attenuation at high frequencies. Such an EMI filter could be widely used in protecting sensitive electronic circuits operating at high frequencies and low voltages.
Additionally, it would be an advance in the art of frequency filtering (e.g. high or low pass filtering, band pass filtering or band reject filtering) to provide filters having an improved response at high frequencies.